The present invention relates to a device for transporting air with the aid of so-called ion-wind or corona-wind.
Such a device according to previously known technique includes in principle an air flow duct and a corona electrode and a target electrode arranged axially spaced from each other in the air flow duct, said target electrode located downstream of the corona electrode, seen in the desired air flow direction.
The corona electrode and the target electrode are connected to a respective terminal of a direct-current voltage source, the design of the corona electrode and the potential difference and distance between the corona electrode and the target electrode being such that a corona discharge occurs at the corona electrode. This corona discharge results in air ions having the same polarity as the corona electrode and possibly also charged so-called aerosols, i.e. solid particles or liquid drops present in the air, said particles or drops being charged upon collision with the charged air ions. The air ions move rapidly, under influence of the electrical field, from the corona electrode to the target electrode, where they relinquish their electric charge and again become re-charged air molecules. During this movement the air ions permanently collide with the non-charged air molecules and thus the electrostatic forces are transferred to these latter air molecules, which are thus drawn in a direction from the corona electrode towards the target electrode, thereby causing an air transport in the shape of a so-called ion-wind or corona-wind through the air flow duct.
Preferred designs of air transporting devices of the type mentioned above are described e.g. in the international patent application PCT/SE85/00236. In air transporting devices of this type the corona electrode can be designed e.g. as a wire-shaped electrode element, said wire-shaped electrode element extending across the air flow duct that has a rectangular or square cross-section, the wire-shaped corona electrode elements being provided transverse to the longitudinal axis of the duct.
Problems related to other types of corona electrode elements and the adherent design of the air flow duct, are of the same character as the problems that are solved by the present invention. Thus the below description of these problems are focused to embodiments having an elongated corona electrode as ion source, said design being the most frequent in practical tests.
As can be learnt from the international patent application mentioned above the efficiency of the air transport is directly dependant on the product of the ion current, i.e. the strength of the corona current and the distance between the corona electrode and the target electrode. Further the ion current should be as evenly distributed as possible across the whole cross-sectional area of the air flow duct. However, it has turned out that the walls of the air flow duct, said walls normally having an insulated inner side and a conducting earthed outer side, exert a disturbing action upon the corona discharge, and thus upon the corona current and the so-called ion-wind.
This screening and disturbing influence causes in practice that ion-wind devices are designed having relatively wide air flow ducts. The consequence of this is that the air flow velocity through the duct is very uneven and unfavourable in that the velocity is high in the center plane of the duct downstream of the corona electrode and a dramatically decreased air flow velocity as the distance to the centre plane increases.
As regards air cleaners designed according to the ion-wind technique this means that inter alia the separating part, i.e. the so-called precipitator, must be given dimensions based on very unfavorable air flow conditions. To use some kind of mechanical restriction of the air flow to achieve an even air flow through the precipitator of previously known devices is out of question from a practical point of view, since there is not sufficient generation of pressure in the device to compensate the drop in pressure that such a mechanical restriction would initiate.
Certain attempts have been made to reduce or eliminate the screening effect of the air flow duct, said attempts being described in the international patent application mentioned above. According to this application so-called exitation electrodes are provided on the inner side of the current carrying surfaces of the air flow duct, the purpose of said exitation electrodes is to improve the conditions for generating the so-called ion-wind in the air flow duct by connecting the exitation electrodes to a suitable voltage.
This method has a limited effect and rather than solving the problems described above said method creates electrically defined conditions in the duct including well defined electrode elements that in their turn have a screening effect in a more defined way than the dielectric surfaces of the duct. Especially in narrow flow ducts this method of solving the problems is not suitable.
The aim of the present invention is among other things to provide an air transporting device using so-called ion-wind or corona-wind, said device being free from the problem discussed above, the aim also being to essentially improve the efficiency of the ion-wind device both as regards the air transport and air cleaning as well as regards the simplicity and safety of operation compared to prior art. There are further aims of the present invention that will be described below.